The invention can be employed to utmost advantage for connection of wells through the body of a coal bed in preparation of underground gas generators for underground gasification of coal fields.
Furthermore, the herein disclosed method of connection of wells can be utilized for increasing the water influx to the wells of water supply installations, as well as for lowering the water level in de-watering mineral deposits and also for water supply.
The disclosed method can likewise be employed for washout of underground salt deposits to create therein underground reservoirs for storage of crude oil, oil products and liquefied gases.
At present, the mineless method of gasification of coal beds is effected by means of wells drilled to the base of the coal bed. To perform gasification of the coal bed, the wells or boreholes thus drilled are connected through the coal bed so that upon connection of the wells throughout the entire extent of the coal bed there should be formed an area of increased permeability for the gaseous media employed for gasification and charged into the wells, i.e. the common unitary gasification channel. The gaseous media in most cases is either air or oxygen-enriched air.
According to one of the known methods of connection of wells, there is effected hydraulic-fracturing of the mineral bed intermediate the wells being connected.
Hydraulic-fracturing of mineral beds is widely utilized in development of oil fields, salt and sulphur fields, as well as at underground gasification of coal.
The technique of hydraulic fracturing of a mineral bed resides in that a liquid under high pressure is pumped into one of the wells which is often called in the art "the source-well", the liquid propagating through the coal bed via the system of natural fissures and pores.
The liquid pumped into the bed creates therein an area of increased pressure which tends to open and expand natural fissures in the bed, and this constitutes the hydraulic-fracturing and results in the creation of hydraulic-fracturing fissures. The hydraulic-fracturing of the bed takes place at a moment when the pressure of the liquid pumped into the bed exceeds the pressure of the overlying strata and the fracture strength of the coal bed. Owing to the formation of hydraulic-fracturing fissures the natural permeability of the bed is considerably increased.
The increased permeability of the bed in underground gasification enables an increase of the volume of gaseous agent pumped into the bed, which is necessary for carrying out the gasification process.
According to a hitherto known method of connection of wells by hydraulic-fracturing of the bed, a liquid is pumped into one of the drilled wells or boreholes, i.e. into the source-well under a pressure sufficient for initiating hydraulic-fracturing fissures, the other wells being left open, and the connection of the source-well being effected with one of the adjacent wells. Upon formation of a hydraulic fracturing fissure the latter is flushed with the liquid.
A shortcoming of the above specified known method is that it does not provide for directioning of the hydraulic fracturing.
This can be explained by the fact that hydraulic-fracturing fissures are formed predominantly in the direction of maximum natural permeability of the coal bed, which permeability is defined by the direction and dimensions of natural fissures in the coal bed. Therefore, more often than not there are formed short channels having arbitrary directions.